1. Field of the Invention
This invention relates to methods of crimping a stent onto a balloon-catheter assembly to increase retention of the stent thereon.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been subjected to angioplasty or valvuloplasty.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. Radial strength and rigidity, therefore, may also be described as, hoop or circumferential strength and rigidity. Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart.
A stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed so that the stent can be radially compressed to allow crimping and radially expanded to allow deployment, as described below.
Additionally, it may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Thus, stents are often fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such that they completely erode only after the clinical need for them has ended.
In general, there are several important aspects in the mechanical behavior of polymers that affect stent design. Polymers tend to have lower strength than metals on a per unit mass basis. Polymeric struts or bar arms in a polymeric stent can crack during crimping and expansion, especially when brittle polymers are used. The portions of the stent subjected to substantial deformation tend to be the most vulnerable to failure. Furthermore, in order to have adequate mechanical strength, polymeric stents may require significantly thicker struts than required in metallic stents, which results in undesirably wider struts in polymeric stents.
Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, inflating the stent at the treatment location, and removing the catheter from the lumen by deflating the balloon. “Delivery” refers to introducing and transporting the stent through a bodily lumen to the treated site in a vessel. “Deployment” corresponds to expanding the stent within the lumen at the treatment site. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon-catheter assembly. The stent must have sufficient retention during delivery prior to deployment of the stent at an implant site. Once the stent and catheter is positioned at an implant site, the stent is then expanded by inflating the balloon.
The balloon-catheter assembly and the attached stent must have a small delivery diameter to be able to be transported through the small diameters of the vascular system. Also, the stent must be firmly attached to the catheter to avoid detachment of the stent before it is delivered and deployed in the lumen of the patient. Detachment of the stent can result in medical complications. For example, a detached stent can act as an embolus that may create thrombosis and require surgical intervention. For this reason, stent retention on the balloon-catheter assembly is important.
What is needed is a method for crimping a polymeric stent to increase stent retention on a delivery balloon, while maintaining mechanical properties of the stent during crimping.